Health Calculators

Health calculators turn a couple of measurements into a number you can compare against a reference range. They are screening aids, not diagnoses, and understanding their limits matters more than the arithmetic.

What BMI is

Body Mass Index is weight in kilograms divided by height in metres squared. The BMI Calculator applies that and places the result in the WHO categories: under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, 30 and above obese. Lower thresholds are recommended for some Asian populations, where elevated risk appears at a lower BMI.

Where it misleads

BMI was devised in the 19th century to describe populations, not individuals, and it takes no account of body composition. Muscle is denser than fat, so athletes routinely register as overweight while carrying very little fat. It says nothing about where fat is stored, even though abdominal fat is a stronger risk indicator than total mass. And it is unreliable for children, during pregnancy, and for older adults who have lost muscle — a child's BMI has to be read against age- and sex-specific percentile charts rather than fixed categories.

Better questions to ask

Waist circumference and waist-to-height ratio capture abdominal fat and correlate more closely with metabolic risk. Keeping your waist under half your height is a simple and widely cited guide. Body fat percentage is more informative still, though it needs proper measurement rather than a formula.

None of this replaces a clinician. A number outside a reference range is a prompt to ask a professional, not a conclusion.